Aluminum extrusions are cleaned, etched and anodized for architectural and home improvement applications. The etching treatment is important in producing a finished product having the desired appearance. In current commercial practice, aluminum extrusions, mainly to alloy specification 6063 (magnesium and silicon), are etched in a caustic soda based solution at temperatures of 50.degree.-70.degree. C. The solution contains additives including sequestering agents, smoothing agents (sulfides) and, often, surfactants, in order to help achieve a uniform satin appearance prior to anodizing. Such solutions often have long life properties and provide a quite uniform finish. Caustic based solutions produce a whitish, satin appearance on the surface which retains a considerable degree of reflectance even after anodizing in sulfuric acid to a thickness of 15-25 micrometers. With the advent of electropainting of aluminum, in particular with a white finish, there is a need to produce a dull, low reflectance, more matte, dense, white or grayish white anodized surface. The surface produced by the caustic soda based etching solutions is not dense enough. Also, the caustic content has to be carefully controlled and the baths are unpleasant to use and the sulfide additions have to be oxidized before discharge.
Solutions containing fluorine compounds are known for cleaning and etching aluminum, for example, as are described in U.S. Pat. No. 4,124,407 which discloses mixtures of HF and H.sub.2 SO.sub.4 for cleaning with little or no etching, U.S. Pat. No. 3,634,262, which discloses perdiphosphate compound and a fluoride salt for preparing surfaces for anodizing, and U.S. Pat. No. 4,230,522, which discloses a mixture of phosphoric acid, nitric acid, acetic acid and a fluoroborate anion containing compound for etching thin films of aluminum. Etchants based on hydrofluoric acid or ammonium bifluoride have been used successfully to produce a very white finish on sheet materials suitable for nameplate applications. When used on 6063 alloy extrusions, however, hydrofluoric acid based solutions act as metallographic etchants which reveal grain structure and other metal faults or structure. I have now found that certain solutions based on dilute hexafluorophosphoric acid provide smooth, white, matte surfaces which are especially suitable prior to electropainting or anodizing.